Behavior of Gases

Chapter 10 & 12

Laws versus Theories

• Scientific Law–A law is a statement that describes

actions or a set of actions that occurs consistently.

–Example: Law of Conservation of Mass

• Scientific Theory–A theory is a model that explains why

experiments give certain results.–Example: Kinetic Molecular Theory

Postulates of the Kinetic Molecular Theory of Gases

• All gases particles are in constant and random motion.

• There are no sources of attractions or repulsion among gas particles.

• The collision of these gas particles with an object results in gas pressure.

• The average kinetic energy of these gas particles is directly related to their absolute temperature.

Table for Data CollectionPage 2

Name of Demonstration or Experiment

ObservationsAn Explanation

Using the Kinetic Molecular

Theory

Universal Indicator Experiment

Squirt Bottle

Can Crushing

Candles in a Beaker

5

Kinetic Molecular Theory Experiment

Chemical Reactions

HCl(aq) + NaHSO3(aq) SO2(g) + NaCl(aq) + H2O(l)

AND

NaOH(aq) + NH4Cl(aq) NaCl(aq) + H2O(l) + NH3(g)

6

Kinetic Molecular Theory Experiment

• BTB–Acidic Solutions: Red or

Yellow/Orange–Basic Solutions: Blue or Purple

• Safety–Wear your safety goggles!

7

Explanation of Kinetic Molecular Theory Experiment

Postulate #1 & 2The universal indicator changed colors

in every direction because gas particles are in constant and random motion, which means the gas should

go in every direction.

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Crushing CanPostulate #3 & 4

• As the water boils, the can becomes full of steam.

• When the can is inverted into the cold water bath, the temperature of the gas inside the can drops and some of the water condenses.

• Since the temperature drops and there are fewer gas particles, the pressure inside the can decreases.

• Since the pressure outside the can is now much greater, this higher pressure crushes the can.

9

Squirt BottlePostulate #3

• Dry ice is changing from a solid to a gas. This creates more gas particles inside the water bottle.

• Since there are more gas particles, there will be more collisions of gas particles with the walls of the container.

• Since there are more collisions of gas particles with the walls of the container, the pressure inside the bottle increases.

• The higher pressure pushes the water up and out of the water bottle. (Remember pressure is a force, which is a push or a pull!)

10

The Crushing Can in Real Life

11

Candles in a Beaker

I want you to record • your observations.• explain why your

observations are occurring using the kinetic molecular theory.

Wear safety goggles!

12

Candles in a Beaker

Postulate #3 & 4• When the candles go out, the

temperature begins to drop.• The gas particles slow down, which

causes the pressure inside the beaker to drop.

• The higher pressure on the outside of the beaker pushes the water into the beaker.

Pressure• What is pressure?

A Force Exerted by a Gas over a Given Area

• What causes pressure?Collisions of the Gas Particles with the

Walls of the Container

Gas Particle

That’s Pressure! Walls of

theContaine

r

Units of Pressure

atmospheres = atmmillimeters of mercury =

mmHgkilopascals = kPa

pounds per square inch = psi

1 atm = 760mmHg = 101.3kPa = 14.7psi

Sample Problem #1

How many kilopascals are equivalent to 880mmHg?

880 mmHgkPa

mmHg760

kPa

mmHg101.3

760

kPa

mmHg

120 kPa117.3 kPa

Sample Problem #2

Calculate the number of psi that are in 2.60atm.

2.60 atm psi

atm1

psi

atm14.7

1

psi

atm

38.22 psi 38.2 psi

Homework

1. Complete pg. 5 in your booklet.

What is held constant?

Pressure

Number of Gas Particles

Graph for Charles’s Law

V

T

Equation for Charles’s Law

Direct

Inverse or Direct?

Charles’s Law

1 2

1 2

V V

T T

Temperature must be in Kelvin.

On the Back of Your Index Card

Summarize Charles’s Law in a single, complete sentence.

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Example Problems

1. The temperature of a 0.65L sample of carbon dioxide gas is 580K. If the pressure remains constant, what is the new volume of the gas if the temperature increases to 1300K?

2. A balloon has a volume of 5.6L at a temperature of 98oC. If the volume of balloon increases to 9.5L, what will be the temperature of the gas in Celsius? Assume that the pressure remains constant.

What is held constant?

Volume

Number of Particles

Graph for G-L’s Law P

T

Equation for G-L’s Law

Direct

Inverse or Direct?

Gay-Lussac’s Law

2

2

1

1

T

P

T

P

Temperature must be in Kelvin.

On the Back of Your Index Card

Summarize Gay-Lussac’s Law in a single, complete sentence.

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Example Problem

A certain gas has a pressure of 56.0kPa at a temperature of 56.1oC. If the volume remains constant, what would be the new pressure if the temperature was increased to 78.2oC?

Homework

Complete pg. 10 & 12 in your booklet.

What is held constant?

Temperature

Number of Gas Particles

Graph for Boyle’s Law P

V

P1V1 = P2V2

Equation for Boyle’s Law

Inverse

Inverse or Direct?

Boyle’s Law

On the Back of Your Index Card

Summarize Boyle’s Law in a single, complete sentence.

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Example Problems

1. The pressure of a 3.5L balloon was determined to be 1.5atm. Assuming that the temperature remained constant, what would be the volume of the balloon if the pressure was decreased to 0.45atm?

2.  At 45oC, a certain container of gas has the volume of 580mL and a pressure of 980mmHg. What would be the new volume of the gas at 250 mmHg and 45oC?

Homework

Complete pg. 14 in your booklet.

Combined Gas Law

1 1 2 2PV PV

1 2

1 2

V V

T T

1 2

1 2

P P

T T

1 1 2 2

1 2

PV PV

T T

Temperature must be in Kelvin.Remember STP = 1 atm and 0oC

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Example Problems

1.A hot air balloon has a volume of 7500L at 270K and a pressure of 1.2atm. What will be the volume of the balloon if the pressure changed to 0.90atm and the temperature decreases to 230K?

2.The volume of a gas at STP is 22.4L. At 12oC, the volume of the balloon changes to 55.0L. What is the new pressure?

What is held constant?

Temperature

Volume

Number of Gas Particles

Picture of Dalton’s Law

Ptotal = P1 + P2 + P3…

Equation for Dalton’s Law

This law only applies to a

mixture of gases.Important Reminder

Dalton’s Law of Partial Pressures

A partial pressure is the pressure a gas would have or would exert if it were alone in the container.

+ =

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Example Problems

1. Air contains oxygen, nitrogen, carbon dioxide, and trace amounts of other gases. What is the partial pressure of oxygen (Poxygen) at 101.3kPa if the partial pressures of nitrogen, carbon dioxide, and other gases are 79.10kPa, 0.040kPa, and 0.94kPa, respectively?

2. A mixture of gases contains oxygen, nitrogen, and helium. The partial pressure of oxygen is 2.1atm. The partial pressure of nitrogen in 0.21atm, and the partial pressure of helium is 7.80atm. Determine the total pressure of this mixture.

Ideal Gas LawDescribing the Behavior of Ideal Gases

P V = n R TP = pressureV = volume in LITERSn = molesR = gas constantT = temperature in KELVIN

The value of R that you use is based on your units of pressure.

R = 0.08206 or 62.4 or 8.314 Kmol

atmL

Kmol

mmHgL

Kmol

kPaL

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Ideal Gases

• We assume that all gases behave ideally. • Most gases are not 100%

ideal.• Gases tend to be the most

ideal at high temperatures and low pressures!

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Example Problems

1. A 5.60 L sample of an ideal gas contains 0.954 moles at 742 mmHg. Calculate the temperature.

2. What volume is occupied by 19.6g of methane, CH4, at 27oC and 1.59atm?

3. What mass of 2250mL chlorine gas at 45oC and 120kPa?

Don’t forget about your diatomic elements!

H2, N2, O2, F2, Cl2, Br2, I2

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Tips for the Ideal Gas Law Problems with Stoichiometry

1. The volume will always need to be in liters and the temperature in Kelvin.

2. Always have a balanced chemical equation!3. You can only use 22.4 L = 1 mol at STP.4. For most problems, you will do one of two

ways.1. Solving for Volume: You will use

stoichiometry to solve for moles and then use the ideal gas law to solve for volume.

2. Solving for Moles or Mass: You will use the ideal gas law to solve for moles and then use stoichiometry to solve for moles or mass.

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More Challenging Gas Law Problems

1. Nitrogen and hydrogen gases react to produce ammonia. Determine the mass of ammonia that would be produce if you reacted 4.0L of nitrogen gas at 298K and 1.5 atm.

2. Water decomposes to form oxygen and hydrogen. If 3.4g of water decomposes, what volume of oxygen gas would be formed at 56oC and 2.3 atm?